1. Field of the Invention
The present invention relates to a method and apparatus for transmitting and receiving feedback information of a user terminal in a communication system including user terminals with different numbers of antennas.
2. Description of the Related Art
With the recent increasing demand for services that transmit the same multimedia data packets to multiple user terminals, communication-broadcasting convergence technologies and services are under scrutiny, which take advantage of merits and make up for defects of communication networks and broadcasting networks. Technologies for improving the efficiency of broadcasting and multicast transmission in the communication network may include Multimedia Broadcast and Multicast Service (MBMS) which has been standardized in 3rd Generation Partnership Project (3GPP), and Multicast and Broadcast Service (MBS) which has been standardized in IEEE 802.16.
MBMS is a service of simultaneously transmitting the same data packets to multiple user terminals. That is, multiple user terminals using MBMS simultaneously transmit the same data packets through one link. MBMS is similar to Cell Broadcast Service (CBS). While CBS is a low-speed message-based service, MBMS is a service aimed at high-speed multimedia data packet transmission based on Internet Protocol (IP) multicast transmission. Advantageously, therefore, MBMS can increase the efficiency of resource use, and user terminals using the same can use multimedia services on the cheap.
MBMS can be divided into multi-cell based MBMS and single-cell based MBMS.
In multi-cell based MBMS, multiple base stations transmit the same data packets based on a Single Frequency Network (SFN). Then, user terminals can obtain a macro diversity gain by soft-combining the data packets received from the multiple base stations. Due to these characteristics, multi-cell based MBMS is advantageous for data packet transmission of a broadcast type that uses only the downlink. Disadvantageously, however, multi-cell based MBMS can hardly guarantee Quality of Service (QoS) on a user terminal basis. Therefore, multi-cell based MBMS is not often used for transmitting different multicast data packets on a region basis.
Single-cell based MBMS has emerged to make up for defects of multi-cell based MBMS, i.e., to guarantee QoS of individual user terminals. Single-cell based MBMS guarantees QoS of each of user terminals included in a multicast group by applying Hybrid Automatic Retransmit ReQust (H-ARQ) or the like. To this end, a base station should receive feedback information from individual user terminals included in the multicast group. The feedback information is an ACK/NACK signal indicating the success/failure in decoding a data packet multicasted from the base station. Accordingly, there is a need for a feedback channel over which the user terminals included in the multicast group can transmit the feedback information to the base station.
An individual allocation method of allocating an ACK/NACK channel for unicast transmission to respective terminals and a shared feedback channel method of using an On-Off Keying (OOK) modulation technique may be considered as a method of allocating the feedback channel. In the shared feedback channel method, user terminals in a group share one feedback channel. Therefore, even if the number of user terminals increases, the required feedback channel resource does not increase.
Meanwhile, it is general that in OOK modulation, an ACK signal is not separately transmitted, since it is considered an Off signal of a NACK signal.
The base station, which has received feedback information, is interested only in determining whether at least one of user terminals included in the multicast group has failed to succeed in decoding the multicasted data packet, rather than in determining whether the individual user terminals have succeeded in decoding the data packet. That is, the base station only needs to know whether there is any user terminal that transmitted a NACK signal, in the multicast group. Therefore, each user terminal does not transmit an ACK signal upon successful decoding, and transmits a NACK signal only upon decoding failure. The base station synthesizes NACK signals that are received from the user terminals through the shared feedback channel, using the OOK modulation technique. The signal synthesized by means of the OOK modulation technique is used to determine the presence of any user terminal that transmitted a NACK signal, using an energy detection technique.
Since the foregoing shared feedback channel method uses one shared feedback channel based on the OOK modulation technique without allocating individual feedback channels to user terminals, there is no additionally required resource despite an increase in the number of user terminals. In addition, since the increase in the number of user terminals increases energy of the synthesized signal, miss detection of a signal may be reduced. Hence, the shared feedback channel method is superior in efficiency compared to the individual allocation method as the number of user terminals in the multicast group increases.
The shared feedback channel method can get a good performance when orthogonal sequences are used in a communication system using multiple antennas. Actually, when the shared feedback channel method is used, user terminals with a single antenna and user terminals with multiple antennas may be used in a mixed way due to, for example, the price issues of user terminals in the cell group. In the worst case, only one user terminal with a single antenna may exist in the group. The base station, however, may not determine whether the one user terminal has a single antenna or multiple antennas. Therefore, the base station requires performance of a specific level or more, just in case of existence of only the user terminal with a single antenna.
Specifically, if a base station constructed to receive feedback information in the form of orthogonal sequences receives the feedback information from only one user terminal with a single antenna, its performance may degrade significantly.
Conversely, even when a base station configured to receive feedback information from only the terminals with a single antenna receives the feedback information from user terminals with multiple antennas, the base station may suffer from performance degradation.